Viruses can be classified in several ways. The most useful is probably by the type of nucleic acid the virus contains, but they can also be classified:
by their geometry
by whether they have envelopes
by the identity of the host organism they can infect
by mode of transmission
by the type of disease they cause
Classification by type of constituent nucleic acid (i.e. by genome type), with examples:
... variety of reasons:
The placebo effect. Medicines or treatments, known to have no effect on a disease, can have major effects on a person's perception of their illness. ...